Polyphasic approach identifies two new species of Desmonostoc (Nostocales, Cyanobacteria) in the coralloid roots of Cycas fairylakea (Cycadales)

Abstract

ABSTRACT Endogenously formed coralloid roots in living cycads are known to be infected and maintained by a mutual partnership with photosynthetic cyanobacteria. In this study, two strains of filamentous, nitrogen-fixing endosymbiotic cyanobacteria from the green zone of the coralloid roots of Cycas fairylakea were isolated and grown in culture. The phylogenetic placement based on the 16S rRNA gene region and the secondary structures of the ITS between the 16S and 23S rRNA gene, in addition to traditional morphological characterization, were used to analyse the two strains. Results showed that the 16S rRNA gene sequences of the two strains, CF01 and CF06, displayed ≥97% similarity to the genus Desmonostoc. The phylogeny inferred by maximum likelihood, bayesian inference and neighbour-joining placed the two strains on a separate node away from other species of Desmonostoc. Hormogonial filaments were evident in the two symbiotic species. In addition, a few unique morphological structures in the CF01 and CF06 strains were observed. The D1-D1ʹ, Box-B and V3 helices generated from the ITS of 16S–23S sequences did not fit those of any described species of Desmonostoc, and this led to proposing the two strains as novel species. In compliance with the International Code of Nomenclature for algae, fungi, and plants, the two species are named Desmonostoc lechangense sp. nov. and Desmonostoc aggregatum sp. nov.